qubes-doc/security/vm-sudo.md
2016-08-26 23:58:02 -07:00

149 lines
6.8 KiB
Markdown

---
layout: doc
title: VM Sudo
permalink: /doc/vm-sudo/
redirect_from:
- /en/doc/vm-sudo/
- /doc/VMSudo/
- /wiki/VMSudo/
---
Password-less root access in VM
===============================
Background ([/etc/sudoers.d/qubes](https://github.com/QubesOS/qubes-core-agent-linux/blob/master/misc/qubes.sudoers) in VM):
user ALL=(ALL) NOPASSWD: ALL
# WTF?! Have you lost your mind?!
#
# In Qubes VMs there is no point in isolating the root account from
# the user account. This is because all the user data is already
# accessible from the user account, so there is no direct benefit for
# the attacker if she could escalate to root (there is even no benefit
# in trying to install some persistent rootkits, as the VM's root
# filesystem modifications are lost upon each start of a VM).
#
# One might argue that some hypothetical attacks against the
# hypervisor or the few daemons/backends in Dom0 (so VM escape
# attacks) most likely would require root access in the VM to trigger
# the attack.
#
# That's true, but mere existence of such a bug in the hypervisor or
# Dom0 that could be exploited by a malicious VM, no matter whether
# requiring user, root, or even kernel access in the VM, would be
# FATAL. In such situation (if there was such a bug in Xen) there
# really is no comforting that: "oh, but the mitigating factor was
# that the attacker needed root in VM!" We're not M$, and we're not
# gonna BS our users that there are mitigating factors in that case,
# and for sure, root/user isolation is not a mitigating factor.
#
# Because, really, if somebody could find and exploit a bug in the Xen
# hypervisor -- so far there have been only one (!) publicly disclosed
# exploitable bug in the Xen hypervisor from a VM, found in 2008,
# incidentally by one of the Qubes developers (RW) -- then it would be
# highly unlikely if that person couldn't also found a user-to-root
# escalation in VM (which as we know from history of UNIX/Linux
# happens all the time).
#
# At the same time allowing for easy user-to-root escalation in a VM
# is simply convenient for users, especially for update installation.
#
# Currently this still doesn't work as expected, because some idotic
# piece of software called PolKit uses own set of policies. We're
# planning to address this in Beta 2. (Why PolKit is an idiocy? Do a
# simple experiment: start 'xinput test' in one xterm, running as
# user, then open some app that uses PolKit and asks for root
# password, e.g. gpk-update-viewer -- observe how all the keystrokes
# with root password you enter into the "secure" PolKit dialog box can
# be seen by the xinput program...)
#
# joanna.
Below is a complete list of configuration made according to the above statement, with (not necessary complete) list of mechanisms depending on each of them:
1. sudo (/etc/sudoers.d/qubes):
user ALL=(ALL) NOPASSWD: ALL
(...)
- easy user->root access (main option for the user)
- qvm-usb (not really working, as of R2)
2. PolicyKit (/etc/polkit-1/rules.d/00-qubes-allow-all.rules):
//allow any action, detailed reasoning in sudoers.d/qubes
polkit.addRule(function(action,subject) { return polkit.Result.YES; });
and /etc/polkit-1/localauthority/50-local.d/qubes-allow-all.pkla:
[Qubes allow all]
Identity=*
Action=*
ResultAny=yes
ResultInactive=yes
ResultActive=yes
- NetworkManager configuration from normal user (nm-applet)
- updates installation (gpk-update-viewer)
- user can use pkexec just like sudo Note: above is needed mostly because Qubes user GUI session isn't treated by PolicyKit/logind as "local" session because of the way in which X server and session is started. Perhaps we will address this issue in the future, but this is really low priority. Patches welcomed anyway.
3. Empty root password
- used for access to 'root' account from text console (xl console) - the only way to access the VM when GUI isn't working
- can be used for easy 'su -' from user to root
Replacing password-less root access with Dom0 user prompt
---------------------------------------------------------
While ITL supports the statement above, some Qubes users may wish to enable
user/root isolation in VMs anyway. We do not support it in any of our packages,
but of course nothing is preventing the user from modifying his or her own
system. A list of steps to do so is provided here **without any guarantee of
safety, accuracy, or completeness. Proceed at your own risk. Do not rely on
this for extra security.**
1. Adding Dom0 "VMAuth" service:
[root@dom0 /]# echo -n "/usr/bin/echo 1" >/etc/qubes-rpc/qubes.VMAuth
[root@dom0 /]# echo -n "$anyvm dom0 ask" >/etc/qubes-rpc/policy/qubes.VMAuth
(Note: any VMs you would like still to have password-less root access (e.g. TemplateVMs) can be specified in the second file with "\<vmname\> dom0 allow")
2. Configuring Fedora TemplateVM to prompt Dom0 for any authorization request:
- In /etc/pam.d/system-auth, replace all lines beginning with "auth" with one line:
auth [success=done default=die] pam_exec.so seteuid /usr/lib/qubes/qrexec-client-vm dom0 qubes.VMAuth /usr/bin/grep -q ^1$
- Require authentication for sudo. Replace the first line of /etc/sudoers.d/qubes with:
user ALL=(ALL) ALL
- Disable PolKit's default-allow behavior:
[root@fedora-20-x64]# rm /etc/polkit-1/rules.d/00-qubes-allow-all.rules
[root@fedora-20-x64]# rm /etc/polkit-1/localauthority/50-local.d/qubes-allow-all.pkla
3. Configuring Debian/Whonix TemplateVM to prompt Dom0 for any authorization request:
- In /etc/pam.d/common-auth, replace all lines beginning with "auth" with one line:
auth [success=done default=die] pam_exec.so seteuid /usr/lib/qubes/qrexec-client-vm dom0 qubes.VMAuth /bin/grep -q ^1$
- Require authentication for sudo. Replace the first line of /etc/sudoers.d/qubes with:
user ALL=(ALL) ALL
- Disable PolKit's default-allow behavior:
[root@debian-8]# rm /etc/polkit-1/rules.d/00-qubes-allow-all.rules
[root@debian-8]# rm /etc/polkit-1/localauthority/50-local.d/qubes-allow-all.pkla
- In /etc/pam.d/su, comment out this line near the bottom of the file:
auth sufficient pam_permit.so
Dom0 password-less root access
------------------------------
There is also password-less user->root access in dom0. As stated in comment in sudo configuration there (different one than VMs one), there is really no point in user/root isolation, because all the user data (and VM management interface) is already accessible from dom0 user level, so there is nothing more to get from dom0 root account.